In recent years, secondary batteries have become important components that are essential as power sources for personal computers, video cameras, cellular phones and the like, or as power sources for automobiles and electric power storage.
Among secondary batteries, lithium ion secondary batteries in particular have the feature of higher capacity density than other secondary batteries, and the ability to operate at higher voltage. They are therefore used in data-related devices and communication devices as secondary batteries that are suitable for size and weight reduction, and development has been progressing in recent years toward lithium ion secondary batteries with high output and high capacity, for electric vehicles or hybrid vehicles that constitute lower public hazards.
Lithium ion secondary batteries or lithium secondary batteries comprise a positive electrode layer and negative electrode layer, with an electrolyte comprising a lithium salt situated between them, where the electrolyte is composed of a nonaqueous liquid or solid. When a nonaqueous liquid electrolyte is used as the electrolyte, the electrolyte solution permeates into the positive electrode layer, readily forming an interface between the positive electrode active material of the positive electrode layer and the electrolyte, so that performance is easily improved. However, since the electrolyte solutions that are in wide use are combustible, it becomes necessary to install safety equipment to minimize temperature increase during short circuiting, or to mount a system for ensuring safety, such as preventing short circuiting. On the other hand, all-solid-state batteries, wherein the liquid electrolyte is replaced with a solid electrolyte to render the entire battery solid, do not employ combustible organic solvents in the batteries, and thus allow safety equipment to be simplified and are considered to be superior in terms of production cost and productivity, and their development is also progressing.
Since the adhesiveness of the positive electrode layer, solid electrolyte layer and negative electrode layer in an all-solid-state battery significantly affects the properties of the battery, such as the energy density, capacity, current density and cycle characteristics, technologies have been proposed whereby confining pressure is applied usually in the direction perpendicular to the stacking surface of the all-solid-state battery, so that adhesiveness of the positive electrode layer, solid electrolyte layer and negative electrode layer is maintained even when deformation or expansion takes place in the all-solid-state battery.
Even in secondary batteries wherein multiple all-solid-state batteries are stacked and electrically connected, the adhesiveness between the multiple all-solid-state batteries often significantly affects the electrical connection between the all-solid-state batteries, and therefore the multiple all-solid-state batteries have confining pressure applied in the direction perpendicular to the stacking surface.
In PTLs 1 to 7 there are described techniques for applying confining pressure to batteries in this manner. For example, PTL 1 discloses a secondary battery with an outer shape having opposing flat surfaces, the opposing flat surfaces being pressed in the charge-discharge state, and a weaker pressure being applied in the non-charge-discharge state than in the charge-discharge state of the secondary battery.